1. Field of the Invention
The present invention relates to a low side switch for driving a load such as a lamp, a light-emitting diode (LED), and an inductor. In particular, the present invention relates to a semiconductor switch capable of reducing a leakage current during an OFF time.
2. Description of the Related Art
Conventionally, as a procedure for driving a load such as a lamp and a coil, a method generally is used for driving a load by turning on/off a switch provided on a low potential side of a load as shown in FIG. 7. In FIG. 7, reference numeral 31 denotes a power supply, 32 denotes a load such as a lamp and a coil, and 30 denotes a switch. As the switch 30, a transistor mainly is used. Among transistors, an N-type power MOSFET, which may be used as a low side switch, mostly is used.
Furthermore, the above-mentioned systems generally are provided with various protection functions. In order to realize a load short protection function, an overcurrent protection function, and the like among the protection functions, it is required to detect a voltage on a low potential side of a load (i.e., a potential of a drain terminal in the case of using a MOSFET as a switch). FIG. 8 shows a conventional example in which a MOSFET is used as a switch, and a function of detecting a voltage of a drain terminal is incorporated.
In FIG. 8, reference numeral 31 denotes a power supply, 32 denotes a load such as a lamp and a coil, and 34 denotes an input terminal of a system. Furthermore, reference numeral 21 denotes a MOSFET that functions as a switch for driving the load 32. Reference numeral 25 denotes a drain electrode of the MOSFET 21, and a potential of the drain electrode 25 is assumed to be VD. Furthermore, reference numeral 24 denotes a gate electrode of the MOSFET 21, and the MOSFET 21 is turned on/off based on a potential of the gate electrode 24. Reference numeral 26 denotes a source electrode of the MOSFET 21, which is grounded. Herein, reference numeral 40 denotes a switching portion, and elements contained therein can be formed on the same semiconductor substrate.
In the switching portion 40, a voltage detection circuit 22 is connected in parallel to the MOSFET 21 between the drain electrode 25 and the source electrode 26. The voltage detection circuit 22 can detect the potential VD of the drain electrode 25 by connecting a resistive element 28 (resistance Ra) and a resistive element 29 (resistance Rb) in series as resistors for detecting a drain voltage. More specifically, the potential VD of the drain electrode 25 can be detected only by monitoring a potential VC of an output signal (voltage detection signal) 23 of the voltage detection circuit 22. The relationship between VD and VC can be expressed as follows:
VC=Rb/(Ra+Rb)xc3x97VD (more specifically, VD=VCxc3x97(Ra+Rb)/Rb)
A drain voltage is detected in this manner, and is used for controlling various functions such as a load short protection function and an overcurrent protection function.
Furthermore, reference numeral 27 denotes a control circuit. The control circuit 27 receives the voltage detection signal VC output from the voltage detection circuit 22 to control the gate electrode 24 of the power MOSFET 21.
However, in the conventional example, even if a leakage current of the power MOSFET 21 is small during an OFF time of the power MOSFET 21, a current flows through the resistive elements 28 and 29 connected between the drain and the source, so that a leakage current flows. In this case, a current of about (power supply voltage)/(total resistance of the load 32 and the resistive elements 28, 29 for detecting a voltage) flows.
When a leakage current is large, a current consumed by the system is increased. Furthermore, in the case where a load is an LED, the LED may emit light even during an OFF time.
In order to minimize a leakage current, a method of increasing a resistance of the resistive elements 28 and 29 for detecting a voltage is considered easily. However, in the case where the voltage detection signal VC is received by a MOSFET or a transistor, when the resistance of the resistive elements 28 and 29 for detecting a voltage is prescribed to be too large, a current flowing as the voltage detection signal VC becomes too small, which may cause inconvenience for control. Furthermore, in order to increase a resistance, a tip area also is increased.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a voltage detection circuit and a semiconductor device in which a leakage current is reduced without increasing a tip area, and various protection functions with respect to a load are ensured.
In order to achieve the above-mentioned object, the voltage detection circuit of the present invention is connected in parallel to a first switching element for controlling a power supply to a load and includes a second switching element and a voltage detedction portion connected in series to each other, wherein the second switching element is connected to a high potential side of the first switching element, and the voltage detection portion detects a voltage of a high potential electrode of the first switching element when the second switching element is conducting.
In the above-mentioned voltage detection circuit, the voltage detection portion is composed of at least two resistive elements, and detects a voltage of the high potential electrode of the first switching element based on a division ratio of the resistance of the resistive elements. In this case, it is preferable that the resistive element is a polysilicon resistor formed on an oxide film of the same substrate as that of the first switching element.
Alternatively, the voltage detection portion is composed of at least two Zener diodes.
Furthermore, the second switching element is composed of an N-channel MOSFET. In this case, it is preferable that a backgate of the N-channel MOSFET is at the same potential as that of a source or at a ground potential. Alternatively, the second switching element is composed of a bipolar transistor or an N-channel MOSFET with a high withstand voltage.
In order to achieve the above-mentioned object, the semiconductor device of the present invention includes: a first switching element for controlling a power supply to a load; the above-mentioned voltage detection circuit; and a control circuit that brings the first switching element into conduction or out of conduction in accordance with a control signal from outside, and brings the first switching element out of conduction based on a voltage detection signal output from the voltage detection circuit.
In the above-mentioned semiconductor device, the second switching element of the voltage detection circuit is brought into conduction or out of conduction in accordance with the control signal from outside.
Furthermore, the first switching element is composed of an N-channel MOSFET, an insulating gate type bipolar transistor, or a bipolar transistor.
Furthermore, it is preferable that the first switching element, the second switching element, and the voltage detection circuit are formed on the same semiconductor substrate.
Furthermore, the first switching element is composed of an N-channel vertical MOSFET with a high withstand voltage, using an N-type silicon substrate as a drain electrode.
Furthermore, the first switching element and the second switching element of the voltage detection circuit are both composed of an N-channel vertical MOSFET with a high withstand voltage, using the same N-type silicon substrate as a drain electrode.
Alternatively, the second switching element of the voltage detection circuit is formed on the same semiconductor substrate as that of the first switching element while being electrically insulated with an insulator from the first switching element.
According to the above-mentioned configuration, a voltage detection circuit is provided so as to be in parallel with a power MOSFET that is a first switching element, in which a voltage detection portion for detecting a drain voltage by dividing a voltage upon a resistance ratio or the like and a second switching element for reducing a leakage current are connected in series. When the power MOSFET is in an OFF state, the second switching element is turned off, and when the power MOSFET is in an ON state, the second switching element is turned on. Consequently, detection of a drain voltage is performed normally when the power MOSFET is in an ON state, and a leakage current can be reduced when the power MOSFET is in an OFF state.
Furthermore, in the voltage detection portion, a plurality of Zener diodes also can be used in addition to voltage division upon a resistance ratio.
Furthermore, a gate electrode of the power MOSFET and a control electrode of the switching element for reducing a leakage current are not connected directly to each other. The control electrode of the switching element for reducing a leakage current directly is supplied with a control signal from outside, and the gate electrode of the power MOSFET is supplied with a control signal via a control circuit that receives a voltage detection signal from the voltage detection circuit. Because of this, when the system is suspended, the power MOSFET and the switching element for reducing a leakage current are both in an OFF state, and when the system is activated, the switching element for reducing a leakage current always is in an ON state. Although the power MOSFET usually is put in an ON state by the control circuit, it is turned off in the case where it is required to perform a protection function based on the voltage detection signal from the voltage detection circuit.
Furthermore, the power MOSFET and the switching element for reducing a leakage current are formed on the same semiconductor substrate. The configuration of the device is as follows: the power MOSFET is a vertical N-channel MOSFET formed on an N-type silicon substrate; the switching element for reducing a leakage current is a vertical N-channel MOSFET formed on the silicon substrate in which the same drain electrode as that of the power MOSFET is used; and a source electrode of the switching element is led out to the surface of the silicon substrate and connected to a resistive element for detecting a drain voltage formed on the same silicon substrate.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.